1. Field of the Invention
This invention relates to devices and methods for monitoring and measuring the movements and the physiological status of living subjects, particularly ambulatory subjects.
2. The Prior Art
Subject-monitoring systems comprise devices and methods that collect data about various aspects of a living subject. Such systems routinely process and communicate thse data to provide useful and usable information about the subject. Two aspects of subject-monitoring systems are of particular interest with respect to the present invention: accelerometry and monitoring physiological status.
A. Accelerometry
Accelerometry is the practice of measuring the acceleration of a body with respect to a reference-frame, which reference-frame may comprise one, two, or three-dimensions depending on the application. Accelerometry plays a vital role in a number of fields, including navigation, transportation, seismology, air-bag technology and aerospace engineering; consequently, the field of accelerometry has benefitted from significant and impressive advances in devices and methods used for measuring acceleration.
The most widely accepted system of describing the movement of a subject in three-dimensional space is to describe the motion with respect to three mutually orthogonal axes—x,y, and z, referred to as Cartesian axes. For each of the three axes, it is possible for the subject to undergo two types of movement: 1) along the axis (translational movement), or 2) about or around the axis (rotational movement). Given two types of movement occurring with respect to three axes, it will be appreciated that in order to fully describe the movement of a subject in three-dimensional space, one must simultaneously consider the motion in all “six degrees of freedom” (6-DOF), in the parlance of the art. If one also wishes precisely to analyze the movement of the subject's limbs and extremities in addition to the movement of the subject as a whole, then the movement of each limb or extremity of interest must be independently subjected to 6-DOF analysis. Obviously, such efforts depend heavily on devices that are capable of sensing acceleration: accelerometers.
Numerous types of accelerometers are available, such as those characterized as solid state (U.S. Pat. No. 6,402,968); piezo-electric (U.S. Pat. No. 6,397,677); magnetic (U.S. Pat. No. 6,427,534); mercury-based (U.S. Pat. No. 3,998,106); fibre optic (U.S. Pat. No. 6,175,108); and pendulum (U.S. Pat. No. 6,422,076). In addition to accelerometers, gyroscopes of various descriptions can be used to measure angular velocities (see U.S. Pat. No. 6,192,756 to Kikuchi et al.).
Accelerometers measure linear or translational acceleration with respect to an axis, referred to as “the axis of measurement” of the accelerometer. The standard accelerometer measures acceleration along one axis of measurement. Such an accelerometer is referred to as “uniaxial.” There are also devices in which multiple accelerometers are aggregated in a common package, which device may then have two (biaxial) or three (triaxial) axes of measurement. As used herein, the term “accelerometer module” refers to an accelerometer device having one or more axes of measurement.
The technology for acquiring 6-DOF data with respect to rigid bodies is employed in a variety of fields, such as automotive engineering and aerospace control systems. In the accelerometry of inert, rigid bodies, the accelerometer modules can be permanently affixed to the object by adhesives, welding, screws, etc., and hence the axes of measurement are predetermined and fixed. This significantly simplifies the computations and techniques for obtaining 6-DOF data. An example is U.S. Pat. No. 6,128,955, granted to Mimura, which teaches how to obtain 6-DOF data with respect to a rigid body such as a vehicle. Mimura's approach is, essentially, to solder three uniaxial accelerometer modules to a rigid plate, oriented so that they sense motion occurring along the vertical, or z, axis. Three additional uniaxial accelerometer modules are attached to the same rigid plate oriented such that they sense motion in arbitrary directions in the x-y plane, which plane is orthogonal to the z-axis. The acceleration data thus collected are then processed to provide 6-DOF data about the movements of the vehicle.
The present invention exploits and improves upon existing accelerometry technology as a means of enhancing subject-monitoring by obtaining and utilizing 6-DOF data. Prior to the present invention, the advantages of 6-DOF accelerometry have not been extended to subject-monitoring; nevertheless, physiology and medicine have benefitted from the availability of uniaxial (1-DOF), biaxial (2-DOF), and triaxial (3-DOF), accelerometer modules. In addition, 4-DOF measurements have been described. For example, U.S. Pat. No. 6,436,052 issued to Nikolic et al., discloses the use of two 2-axis solid state accelerometer modules attached to a subject to monitor and measure 4-DOF acceleration in the x-y plane. From the raw data so obtained it is possible, according to the computational methods disclosed by Nikolic, to derive approximate rates of oxygen consumption, and, hence, the amount of work done by the subject. Such applications of accelerometer modules to monitor the activity of a subject are fairly common, and additional examples may be found in U.S. Pat. Nos. 6,306,088; 5,573,013; and 6,307,481.
Pedometers, a sub-set of this art-field, generally employ one or more uniaxial accelerometer modules or motion detectors attached to one or more of a subject's body-segments. By employing such pedometers, information regarding steps taken, cadence, distance travelled, and energy expenditure may be obtained. The invention of Takenaka, disclosed in U.S. Pat. No. 6,254,513, is a representative example of how elementary accelerometer-based pedometers can be employed in the art of measuring human movement. Because existing pedometer devices do not utilize 6-DOF technology, they do not provide information regarding rotational movements and other three-dimensional aspects of gait that must be determined in order fully to analyze a subject's gait.
B. Physiological Monitoring
Predictions of high rates of increase in the numbers of elderly persons requiring care in the coming decades have become quite common. For instance, Petenlenz et al (U.S. Pat. No. 6,433,690) have cited statistics predicting the total number of falls that elderly persons will experience over the coming decades. Such statistics indicate that the amount of care the elderly will require over the next forty years is increasing at an alarming rate. Although the areas to which the present invention can be applied extend well beyond the care of the elderly, the field of patient and elderly care is an especially fertile source of relevant art because the present invention relates to devices and means for obtaining information related to the motion, position, and orientation of a subject in three-dimensional space, in combination with information indicative of the subject's physiological status—information that is often sought by those caring for aged persons.
Although the field is far too large to inventory here, a representative example of devices and methods that are used to monitor the physiological status of patients is U.S. Pat. No. 6,102,856, issued to Groff et al, who disclose a monitoring-system that collects, analyzes and transmits information related to the vital signs of a patient. Another such representative device is disclosed by U.S. Pat. No. 6,215,403 to Chan et al. which discloses a complex of sensors for monitoring a variety of physiological parameters simultaneously. Typically, such devices utilize readily-available sensors that detect and measure physiological parameters and functions such as body temperature, heart rate, cardiac electrical activity, and respiratory function.
The relevant prior art teaches how to use such sensors to collect physiological data, process the data, and transmit them to a monitoring device, or activate some sort of alarm when the parameters being measured indicate an abnormal and/or detrimental condition, such as a precipitous fall in heart rate. When such physiological monitoring systems are combined with accelerometers, one may simultaneously monitor a subject's physiological status and his/her movements, orientation, and position in three-dimensional space. Cherry et al. (U.S. Pat. No. 5,701,894) discloses one such invention combining elementary accelerometry and physiological monitoring.
As discussed above, existing subject-monitoring devices and methods do not have a 6-DOF capability; hence, they can provide, at best, only an incomplete analysis of a subject's movement in three-dimensional space. Nevertheless, such devices can roughly determine, for example, whether the subject is horizontal or vertical, or whether he/she is changing from vertical to horizontal. When such information is combined with data regarding physiological status, a useful impression, even if somewhat imprecise, emerges as to the subject's overall condition. This technology would benefit significantly by 6-DOF measurement capabilities.